For many years limited rotation motor devices of various types have been used. Some utilize the principle of a moving coil of the d'Arsonval type while others utilize the principle of a moving iron. One function of these limited rotation motors is to move a stylus or pen. Others are used in conjunction with a mirror or laser to deflect a beam of light as in a high speed oscillograph or with various types of office copying machines. In all of these devices there exists the need to enhance accuracy and to control "overshoot" when a transient signal is applied to the device.
Accuracy may be enhanced by means of the addition of a closed loop servo transducer and appropriate electronic circuitry. In these closed loop servo systems a transducer is used to sense the position of the rotor of the limited rotation motor. Transducers such as strain gages, linear variable transformers, capacitors and variable resistors are used and some of the disadvantages of these methods are as follows: Linear variable differential transformers are generally bulky and they require an excitation signal; moreover they rely for accuracy on the linearity of a spring system. Strain gages require additional circuitry and they are difficult to calibrate and balance and are affected by temperature changes. Capacitor type transducers rely on an air gap which is subject to contamination; they require extensive circuitry to calibrate and balance and they are subject to temperature variations. Recent improvements in the materials used to manufacture variable resistors, make them good potential transducer devices but they are difficult to construct so as to be free from non-linearity and are subject to vibrations generated between the pick-off element and the resistive material.
The present invention describes an improved limited rotation motor utilizing a variable resistive transducer mechanism in a closed loop servo system. Specifically the invention relates to a closed loop servo system in which a moving coil galvanometer is utilized. The invention as such has particular utility as the drive mechanism for a recording chart wherein an electrical input from the source such as the vital signs of a medical patient is translated into a graphic record. Obviously, other than medical information may be recorded, however, such has particular applicability to the present invention.
A particularly desirable motor movement for the translation of such input signals into usable mechanical movements is that based on the d'Arsonval principle in which a moving coil of a galvanometer enables limited rotation to be effected such that a heated stylus or other such device attached to the supporting shaft of such moving coil may translate such limited rotational movement into a permanent chart recording. A publication (copy attached) generally describing this and other type motor movements is entitled "Chart Recorders" and found on pages 44-46 and 49 of New Electronics (Mar. 5, 1974). A problem with the use of such device is, however, that the input signal to the coil which in effect moves the coil in the above-discussed limited rotational manner, inherently causes the coil to rotate a greater or lesser distance than the proportional strength of the input signal due to its mechanical inertia. In order to overcome this effect, it is possible to have the coil operate against a spring load, however, such springs introduce forces which must be initially overcome and further have a tendency not to restore the coil to its proper position, and thus may create a back-and-forth vibrational type effect before damping the correct position of the coil which is truly representative of the input signal is arrived at.
In order to eliminate such problem, a servo feedback or null circuit may be utilized. Therein a contact arm connected to a hub in turn connected to the shaft of the moving coil and accordingly movable therewith may be used to create a feedback signal proportional to the physical position of the arm vis-a-vis a resistance track representative of the full travel range of such coil. The feedback or correction created from such contact between the arm and resistance track is then compared with the input voltage and correction of the rotational position of the shaft immediately and continuously made. The manner in which such arm moves can, however, cause inaccuracies by introducing undesirable transients into the system.
Accordingly, a primary object of the present invention is the provision of a closed loop servo system utilizing a rotational resistance element as the feedback sensing device of a galvanometer and which is not subject to undesirable effects such as mechanically or electrically induced transients.
These and other objects of the present invention are achieved by the provision of a closed loop servo system including a galvanometer having a shaft mounted coil arcuately movable within maximum rotational limits through a magnetic field by means of a driving input signal thereto. A hub connected to the shaft of the moving coil and in turn rotatable therewith supports a spring biased contact arm which is adapted to move across a resistance track representative of the total possible coil movement. The movement creates the feedback signal which is compared with the input signal so as to rapidly determine whether or not the input signal has moved the coil to its proper position. Viscous damping material is introduced between the connection area of the arm to the hub such that this material creates a viscous dampening effect thereon so as to eliminate or reduce undesirable vibrational movement of the contact arm.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.